Update example of "with any role" method in README.md to avoid ambiguity

README.md is ambiguous by stating that `Forum.with_any_role(:user, :admin)` when Forum is a `resourcify` model.

I've got `undefined method 'with_any_role' ` while trying to do `Forum.with_any_role(:user, :admin)`. Not sure if this is a bug. Close this PR if it is.

Thanks.